Diffusion in β2 Fe-Ni-Al alloys

Diffusion in the β(bcc) phase field of the Fe-Ni-Al system was investigated at 1004°C with solid-solid diffusion couples assembled with β₂ alloys of selected composition. Experimental diffusion paths were determined for all couples and interdiffusion coefficients calculated at composition points corresponding to intersections of diffusion paths and maxima and minima of concentration profiles. The dependence of interdiffusion coefficients on composition was most clearly presented in terms of the parameter Fe/(Fe + Ni). The diffusive interactions between aluminum and nickel as represented by the cross coefficients were either positive or negative depending on the ternary composition. The Fe/(Fe + Ni) ratio appeared to be a significant parameter since iron and nickel atoms behave differently in affecting the degree of ordering in nonstoichiometric (Fe, Ni)Al alloys with less than 50 at. pct aluminum.     

Diffusion studies in the β (B2), β′ (bcc), and γ (fcc) Fe-Ni-Al alloys at 1000 °C

Diffusion studies were carried out in the Fe-Ni-Al system at 1000 °C with solid-solid diffusion couples assembled with β (B₂), β′ (bcc), and γ (fcc) single-phase alloys for the development of diffusion structures, diffusion paths, and for the determination of interdiffusion and intrinsic diffusion coefficients. The diffusion structures were examined by optical and scanning electron microscopy, and the concentration profiles were determined by electron microprobe analysis. Diffusion couples included several series of β vs γ and β′ vs γ diffusion couples characterized by a common terminal alloy bonded to several terminal alloys with varying compositions. The development of planar and nonplanar interfaces, as well as two-phase layers, as observed in various couples, were related to the diffusion paths. The interdiffusion fluxes of individual components were calculated directly from the experimental concentration profiles, and the diffusional interactions among components were examined in the light of zero-flux planes (ZFPs) and flux reversals, which were identified in several couples. Ternary interdiffusion coefficients ( (i, j = Al, Ni)), with Fe considered as the dependent concentration variable, were evaluated at composition points of the intersection of diffusion paths of single-phase couples and of multiphase couples that developed planar interfaces. The interdiffusion coefficients were the largest in magnitude for the β′ alloys, especially near the β/β′ miscibility gap, and decreased for the β and γ alloys. In the β and γ phases, the main interdiffusion coefficient for Al was larger than those for Ni and Fe. Also, Fe interdiffused faster than Ni in the Fe-rich β and β′ phases. The cross-interdiffusion coefficients ( and ) were negative in all three phases. In general, the coefficients were larger in magnitude than the coefficients; however, the magnitude of was greater than that of near the β/(β + γ) phase boundary on the ternary isotherm. In the β phase, the magnitude of (i, j=Al, Ni) coefficients increased over 1 to 2 orders of magnitude with a decrease in the Al concentration and increase in the Fe/Ni concentration ratio. Interdiffusion coefficients, extrapolated from the ternary coefficients for binary alloys, were consistent with those in literature. Intrinsic diffusion coefficients were also determined at selected compositions. In addition, tracer diffusion coefficients were estimated for the binary Fe-Al and Ni-Al alloys at selected compositions, from an extrapolation of ternary interdiffusion coefficients.     

Multiphase diffusion in Fe−Ni−Al system at 1000°C: II. Interdiffusion coefficients for β and γ alloys

Ternary interdiffusion coefficients were determined at 1000°C at several Fe−Ni−Al alloy compositions with multiphase β(bcc)vs γ (fcc) diffusion couples which developed planar β/γ-interfaces. The coefficients, (i,j=Al or Ni) were calculated at compositions corresponding to points of intersections of diffusion paths with Fe taken as the dependent component. These coefficients varied with composition by 1 to 2 orders of magnitude in the β-phase but relatively little in the γ-phase. Empirical relations were derived to describe the composition dependence of the main coefficients. and . Interdiffusion coefficients with either Al or Ni as the dependent component were also evaluated. The relative diffusivities of the elements increase in the order, Fe, Ni, Al for both β- and γ-alloys. The ternary diffusion data were consistent with binary interdiffusion coefficients for Fe−Al and Fe−Ni alloys. G. H. CHENG, formerly a Graduate Student at Purdue University     

Interdiffusion in the Ni-Cr-Co-Mo system at 1300 °C

Interdiffusion was investigated with solid-solid diffusion couples in theα (fcc) region of the quaternary Ni-Cr-Co-Mo system at 1300 °C for the determination of diffusion paths and diffusional interactions among the components. The concentration profiles for a given couple exhibited a common cross-over composition, Y_c, which reflected the relative depths of diffusion in the terminal alloys. Interdiffusion fluxes were calculated directly from the concentration profiles, and the quaternary interdiffusion coefficients were calculated at selected compositions. Ni and Co exhibited uphill diffusion against their individual concentration gradients in a direction opposite to the interdiffusion of Cr. Quaternary diffusion paths were presented as a set of partial diffusion paths on the basis of relative concentration variables.     

Reaction diffusion in the NiCrAl and CoCrAl systems

The oxidation life of a physically vapor-deposited overlay protective coating based on the Ni-Cr-Al or Co-Cr-Al (M-Cr-Al) systems is controlled by aluminum consumption resulting from alumina spalling, erosion, and interdiffusion with the substrate. The rates of these processes are determined by coating and substrate composition, service environment, and temperature. The purposes of this study were 1) to determine the effect of coating and substrate chemistry on the kinetics of interdiffusion and their relation to diffusion-zone constitution and 2) to develop a procedure for analyzing diffusion in multicomponent, multiphase systems. Semi-infinite diffusion couples with MCrAl sources representative of coatings and sinks representative of nickel-or cobalt-base gas turbine alloys were annealed at 1000, 1095, 1150, or 1205°C for as long as 500 h. The couples were examined by optical microscopy and layer growth measurements were made. Parabolic growth constants can be correlated with source aluminum content for a specific sink composition since changes in source aluminum content are more important than changes in chromium content. Sink composition is as important as source composition in determining β-recession kinetics in diffusion couples and life in finite coatings. Nickel-base alloys are more stable sources and stronger sinks than cobalt-base alloys. Total and diffusion activation energies were determined from layer growth constants. By introducing the concept of β source strength which can be determined from the appropriate phase diagrams, the Wagner solution for consumption of a second phase in a semiinfinite couple was successfully applied to the analysis of the MCrAl couples. This provided a method 1) for correlating β-recession rate constants with couple composition, 2) for determining reliable total and diffusional activation energies, and 3) for calculating interdiffusion coefficients.     

Diffusion in ternary Ag-Zn-Cd solid solutions

Diffusion in the Ag-Zn-Cd system was investigated at 600°C with semi-infinite, vapor-solid couples in order to evaluate intrinsic diffusion coefficients and interdiffusion coefficients in theα andβ phases. Intrinsic diffusion coefficients were determined at several composition points in theα-phase region along concentration lines of 11 at. pct Zn, 18 at. pct Zn, 5 at. pct Cd, and 9 at. pct Cd. Interdiffusion coefficients in theα phase were determined at several points of intersection of various composition paths; these composition points of intersection were along lines of constant zinc-to-cadmium concentration ratios of 3.8, 1.8, and 1.2. All two-phase diffusion couples developed planar β/α interfaces. Intrinsic coefficients were computed at three composition points in the β region corresponding to a silver concentration of approximately 55 at. pct. Intrinsic coefficients for β alloys are two to three orders of magnitude larger than those for α alloys. P. T. CARLSON, formerly Donnan Fellow, School of Materials Science and Metallurgical Engineering, Purdue University, Lafayette, Ind., This paper is based on a thesis submitted by P. T. CARLSON in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Purdue University.     

Zero-flux planes and flux reversals in Cu−Ni−Zn diffusion couples

Concentration profiles of isothermal diffusion couples in binary as well as multicomponent systems can be analyzed directly for interdiffusion fluxes without the need for a prior knowledge of interdiffusion coefficients. Such an analysis is presented and applied for the calculation of interdiffusion fluxes of each component at various sections of several diffusion couples in the Cu−Ni−Zn system investigated at 775°C. A major outcome of these calculations is the identification of “zero-flux planes” for the individual components within the diffusion zones of ternary couples. At a zero-flux plane the interdiffusion flux of a component goes to zero and on either side of the plane occurs a change or reversal in the direction of the interdiffusion flux of the component. The formation as well as the number of zero-flux planes of the components is dictated by the terminal alloys of the diffusion comple. The compositions of zero-flux planes for Ni and Cu identified in several Cu−Ni−Zn couples are found to correspond to composition points of intersection of diffusion paths and isoactivity lines drawn through the terminal alloys of the couples on a ternary isotherm. C. W. KIM, formerly a graduate student at Purdue University     

Zero-flux planes and flux reversals in the Cu- Ni- Zn System at 775 °C

Ternary diffusion in the Cu-Ni-Zn system was investigated at 775 °C for the development of zero-flux planes (ZFP) and flux reversals of the individual components. ZFP’s, where the interdiffusion flux of either Cu, Ni, or Zn goes to zero, were identified in several series of single phase and multiphase solid-solid diffusion couples assembled with a (fcc),β (bcc), or γ (cubic) Cu-Ni-Zn alloys and characterized by terminal alloys of similar thermodynamic activity for one of the components. Profiles of interdiffusion fluxes were directly determined from concentration profiles. The diffusion path for a single phase couple with a ZFP was experimentally found to be invariant with diffusion time. The locations of ZFP’s within the diffusion zone of a couple corresponded to sections where the activity of a component was the same as its activity in either of the terminal alloys of the couple. Couples developing ZFP’s showed regions where a component diffused up its own activity gradient. The diffusional interactions among the components described by the ratios of cross to main ternary interdiffusion coefficients were determined directly from the slopes of the diffusion paths at various ZFP compositions. In several multiphase couples, discontinuous flux reversals for the components were also identified at theβ/a and γ/β interfaces. A discontinuous flux reversal for a component was observed at a planar interface, when the activity of the component at the interface corresponded to its activity in one of the terminal alloys of the couple.     

A new analysis for the determination of ternary interdiffusion coefficients from a single diffusion couple

Concentration profiles that develop in a ternary diffusion couple during an isothermal anneal can be analyzed directly for average ternary interdiffusion coefficients. A new analysis is presented for the determination of average values for the main and cross-interdiffusion coefficients over selected regions in the diffusion zone from an integration of interdiffusion fluxes, which are calculated directly from experimental concentration profiles. The analysis is applied to selected isothermal diffusion couples investigated with α (fcc) Cu-Ni-Zn alloys at 775 °C, β (bcc) Fe-Ni-Al alloys at 1000 °C, and γ (fcc) Ni-Cr-Al alloys at 1100 °C. Average ternary interdiffusion coefficients treated as constants are calculated over composition ranges on either side of the Matano plane and examined for the diffusional interactions among the diffusing components. The ternary interdiffusion coefficients determined from the new analysis are observed to be consistent with those determined by the Boltzmann-Matano analysis at selected compositions in the diffusion zone. The ternary interdiffusion coefficients are also employed in analytical solutions based on error functions for the generation of concentration profiles for the selected diffusion couples. The generated profiles are a good representation of the experimental profiles including those exhibiting uphill diffusion or zero-flux plane (ZFP) development for the individual components. Uncertainties in the values of the interdiffusion coefficients calculated on the basis of the new analysis are found to be minimal.     

Diffusional and thermodynamic interactions in the Cu-Ni-Zn system at 775°C

Diffusion was investigated in both α(fcc) and β(bcc) phase regions of the Cu-Ni-Zn system at 775°C with solid-solid diffusion couples and interdiffusion coefficients were determined at several compositions. Intrinsic and interdiffusion coefficients were also estimated from available data on thermodynamic activities and tracer diffusivities for α Cu-Ni-Zn alloys; and the estimated coefficients were consistent with those experimentally determined. Large off-diagonal coefficients indicating strong interactions among the diffusing species were observed and could be appreciated in terms of the compositional dependence of the thermodynamic activities of the components.